I have participated in a number of hackathons, particularly when I was at Harvey Mudd College. There are three projects worth mentioning, all for a 12-hour hardware hackathon called MuddHacks. The projects are summarized here.

HandleBar

This was a backpack-mounted bartending/drink dispensing robot. It featured bladders and pumps in the backpack, with tubes running over the shoulder, down the arm, and into a glove. Nozzles extended past the fingertips, so that the wearer only had to bend down a finger and the selected liquid would be dispensed.

Ballbot

A ball-balancing robot that balanced on a basketball. This robot was not completed in the twelve hours of the hackathon because we inadvertently fried our accelerometer, and could not source a new part in time--one of the big difficulties with a hardware hackathon!

Tesla Coil

A MOSFET-driven Tesla coil, designed to be able to play a tune from a MIDI library. Working with the high-voltage electronics was a bit hair-raising, but fun!